Hot-wire bridge overspeed controller



Dec. 21, 1948. G. L. BORELL 2,456,765

HOT WIRE BRIDGE OVERSPEED CONTROLLER Filed April 18, 1945 2 Sheets-Sheet 1 cor/mourn 001.5 :17 A /3 1/ #1 'flHPLIF/EI? INVENTO/f j; o GEORGE L. BO/TELL, 427

Filed April 18, 1945 Dec. 21, 1948.

G. L. BORELL HOT WIRE BRIDGE OVERSPEED CONTROLLER 2 Sheets-Sheet 2 Patented Dec. 21, 1948 HOT-WIRE BRIDGE OVERSPEED CONTROLLER George L. Borell, Minneapolis,

Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application April 18, 1945, Serial No. 589,078 13 Claims. (Cl. 230-11) This invention relates generally to improvements in control systems for the turbine operated supercharging compressors used in connection with internal combustion engines, such as those used in aircraft, and more particularly to means for controlling the speed of the turbine-compressor unit, commonly called the turbo-supercharger.

In such installations, the turbine, which is driven by exhaust gases from the engine, operates at times at very high speed, in order that it may-run the supercharging compressor at a velocity sumcient to provide adequate air for combustion in the engine. As an example, at the higher altitudes where the atmosphere pressure is low, the supercharger must provide very great compression in order to supply air at sufficient pressure for full power in the engine. The safe operating speed of the turbo-supercharger is, of

course, limited for mechanical reasons and any increase above a safe maximum value may cause failure of the parts.

The speed of the turbo-supercharger, and the rate of compression of the air supplied to the engine, is controlled in such systems by a valve member, conveniently termed the waste gate, the position of which controls the flow of exhaust gasesto the turbine. To position this waste gate, in accordance with demand, an electronic amplifier is used and the signal potential applied thereto is regulated by a compound electrical network having several variable factors, responsive to various conditions affecting proper operation of the turbo-supercharger and functioning mainly to select a waste gate position such as to maintain a desired turbo discharge pressure,

The primary object of my invention is to provide an almost completely electrical means for limiting the velocity of the turbo-supercharger and which will operate quickly, positively and with little likelihood of becoming inoperative, to quickly counteract any tendency toward overspeeding.

Another object is to provide electrical means, responsive to the speed of the turbine, for introducing a signal component in the amplifier such as to cause the same to adjust the waste gate to hold theturbine speed at or below the safe maximum value regardless of the call of other parts of the control system for increased speed.

Another object is to provide a combination thermally and electrically responsive control means operative when the turbine speed reaches a predetermined value, to immediately introduce a correction signal to the amplifier such as to through a compressor l2,

eflect the reduction in such speed, but to respond more slowly to the reduction in speed so that the correction signal will not be removed so quickly as to cause immediate restoration of the speed to the same value, such as would cause a hunting action in the control system.

These and other more detailed and specific objects will be disclosed in the course of the following specification, reference being had to the accompanying drawings, in which Figure l is a diagrammatical view of a complete engine induction and exhaust system, embodying a turbo-supercharger and an electrical control system therefor, and showing one modification of my overspeed control;

Figure 2 is a sectional view, partially in elevation, of one suitable arrangement and layout of the thermally responsive resistance elements of the bridge or network shown in Figure 1, and

Figure 3 is a fragmentary diagrammatical viewsimilar to Figure l, but showing a modification of my overspeed limiting network, and showing only pertinent parts of the control system and engine assembly.

Frame 1 Referring now more particularly to Figure 1, I show therein an engine III which may be the engine of an aircraft. Air for supporting combustion in the engine passes from an intake ll a conduit l3, an aftercooler H, a conduit IS, a throttle iii, a carburetor H, a conduit 18, a direct driven compressor ii, a conduit 20, and an intake manifold 2! into the engine.

The exhaust gases from the engine issue from an exhaust manifold 22 and are discharged through a duct 23 having a branch 24 leading to a turbine 25. The turbine is provided with an outlet 26 through which the exhaust gases may escape to atmosphere after passing through the turbine. The conduit 23, commonly termed the exhaust stack, also hasan outlet 21 to atmosphere, and controlling the flow of exhaust gases from this outlet is a damper or valve 28 known as the waste gate.

The compressor i2 is driven from the turbine 25 through a shaft 29 and the air compressed in this compressor 12 passes through the aftercooler I, wherein the heat of compression is at least partly removed by passing fresh air from an intake 30 in heat exchanging relation with the compressor discharge air, after which the cooling air is delivered at the outlet 3|.

The thottle I6 may be adiusted by the hand control 82. and in the carburetor ll fuel from a supply (not here shown) is mixed with the air to form a combustible gas.

The compressor I8 is directly driven from the engine by shaft 88 and is utilized not only as a compressor, but also to evenly distribute the mixture of fuel and air to all cylinders of the engine.

The waste gate 28 is driven by a motor 84 through a gear train 85. The motor 34 is of the split phase type, being provided with a pair of iield windings 38 and 31 which are spaced 90 electrical degrees apart, and an armature 38. The field winding 31 is supplied with electrical energy from a secondary winding 39 of a transformer 48 having a primary winding 4| which is connected to a suitable source (not shown) of alternating current. The energizing circuit for the winding 81 may be traced from the upper terminal of the transformer winding 38 through a conductor 42, a condenser 43, the field winding 81, and a conductor 44 to the lower terminal of the secondary winding 89.

The flow of electrical energy to the motor field winding 88 is controlled by an electronic ampliher 45 to which the winding is connected by conductors 48 and 41. The amplifier 48 is powered from another secondary winding 48 on the transiormer 48, to which the amplifier is connected through conductors 48 and 58.

The amplifier 46 is provided with a pair of input terminals and 52 and operates to supply the field winding 38 of the motor with alternat ing current, the phase of which depends upon the phase of an alternating current signal impressed across these input terminals 5| and 52. Any suitableform of amplifier in which there is a constant phase relation between the input and output voltages may be used. Such amplifier is one of the type shown in Figure 1 of the copending application of Albert P. Upton, Serial No. 437,561, filed April 3, 1942, now matured into Patent No. 2,423,534 of July 8, 1947.

It will be evident that if the motor field winding 38 is supplied with alternating current which leads the current in the other field winding 31 by ninety electrical degrees the motor will rotate in one direction, whereas if the current in winding 88 lags the current in winding 31 by this amount, the motor will rotate in the opposite direction. The condenser 43 is provided for shifting the phase of the current to field winding 81 ninety degrees with respect to what it otherwise would be so that the current supplied to winding 38 by amplifier 45 either leads or lags that supplied to winding 31 by ninety degrees.

The phase oi the signal applied to the ampli fler input terminals SI and 52 is determined by the electrical conditions existing in a compound. network which consists of four main separate networks connected in series. The circuit between the amplifier input terminals may be traced i'rom terminal 5| through a conductor 53, a first electrical network 54, a conductor 55, a second electrical network A forming the primary portion of the subject matter of the present invention, a conductor B, a third electrical network 58, a conductor 51, a fourth electrical network 88, and a conductor 58, back to the input terminal 82.

The network 58 includes a transformer secondary winding 88 across the terminals of which is connected a slidewire resistance 6| by means of conductors 62 and 88. The conductor 59 is connected to a slider 64 which cooperates with the resistance 4| and which is adjustable thereacross by means of a control knob 88. The resistance II and slider 84 together form a control point adjuster 88 or manual selector for the turbine discharge pressure control system.

Another slidewire resistance 61 is connected across one half of the transformer secondary winding 88 by a conductor 88, connected to a lower terminal thereof, and conductors 88 and I. connected to a center tap on the winding. A slider 18 cooperates with ,the resistance 81 to form a calibrating potentiometer designated generally at H. The center tap of the winding ill is connected by conductor 12 to the center of the resistance 8| to decrease the impedance between the sliders 84 and I8.

The network 58 comprises a transformer secondary winding I3 across the terminals of which is connected a slidewire resistance 14 by means of conductors l5 and 18. Cooperating with this resistance 14 is a slider 17 and the conductor 51 connects sliders l8 and I1. The slider 71 and the resistance 14 together form a main pressure controller designated generally at 18, and this controller is adjusted in accordance with the pressure in the induction system of the engine. For this purpose a pressure takeoff duct 19 is connected to any suitable point between the supercharging compressor l2 and the engine, and it is here shown as connected to the duct l5 leading to the throttle i8. The duct 19 conveys the air pressure to the interior of a bellows 88. A second bellows 8i is provided and the two bellows 88 and 8| are supported with their free ends extending toward each other at opposite sides of the slider 11. These free ends are connected by link 82 to each other and to the slider ll so that expansion and contraction of the bellows 88 in response to fluctuations in air pressure therein will shift the slider along the resistance 14. The bellows 8| is evacuated and compensates this controller for variations in atmospheric pressure in a well known manner.

A second slidewire resistance 83 is also connected across the winding 73 by means of conductors 84 and 85, and cooperating with resistance 83 is a slider 86 which is positioned in accordance with variations in the rate of acceleration of the turbine 25. For this purpose there is schematically shown a control device 81 connected to the operating shaft of the turbine, and the resistance 83 and slider 86 together form an acceleration compensating controller 88. it may here be noted that the slider 86 will normally remain in the position shown upon a dead spot 89 at the left hand end of resistance 83, but will swing to the. right upon the acceleration of the turbine beyond a safe value. The dead spot 88 is provided so that minor accelerations of the turbine will have no effect upon the system.

The network 54 includes a transformer secondary winding 88 across whose terminals slidewire resistance 8! is connected by conductors 82 and 83. Cooperating with the resistance is a slider 84 which is moved along the resistance by a connection, as indicated, to the aforesaid gear train 38, the slider movement being thus concurrent with that of the waste gate 28. The resistance 8| and slider 84 together form a follow-up potentiometer 88 and the conductor 58 is connected to the slider.

As thus far described the system is substantially conventional in form. being quite similar to that disclosed in Figure 5 of the copending applicaance conditions ascends latter beingpositioned by a mechanical controller operated of! the turbine shaft 20 to affect balin this network upon the occurrence of an overspeed condition.

.In accordance with the present invention. the network A is substituted for the mechanically positioned overspeed responsive controller of the previous system, this network being connected as indicated'in series with the networks 04 and In the embodiment here illustrated, the network A comprises-a transformer secondary winding across whose terminals are connected the opposite ends of two series connected resistances 01 and 90, by means of conductors 00 and I00. The other ends of said resistances 01 and 00 are connected by a conductor IM to which the conductor B leading to the network 00 is connected. Also connected across the terminals of the secondary winding 90 is a slidewire resistance I02, by conductors I00 and I04, and cooperating with this resistance is a slider I to which conductor 55 is connected. The resistances 01, 00, and I02 form a Wheatstone bridge energized by the secondary winding 86 and the resistance I02 and slider I00 cooperate to form a balancing potentiometer I00 which may be adjusted to balance the bridge.

The resistances 01 and 00 are of the type having a high temperature coeillcient of resistance and have the property of increasing in effective resistanceupon their being heated. The resistance 90 is arranged adjacent to an electrical heating element I01 and together therewith is enclosed in a heat insulating enclosure indicated diagrammatically at I08. This'assembly of the resistance 90, heating element I01 and heat insulating enclosure I00, along with the resistance 01 forming the opposite leg at this side of the bridge, are enclosed within a heat dissipating enclosure or shield indicated diagrammatically at I09, so that resistance 01 is subject to ambient temperatures about the member I00.

The heating element I01 is energized by a transformer secondary winding IIO to one terminal of which one end of the element is connected by a conductor I I I. While an alternating current source such as transformer I20-I I0 is shown as energizing the heating element I01, it is to be understood that a direct current source may be employed if desired. A centrifugally operated speed responsive switch III controls the energization of the heating element I01 and as here shown comprises a disk I I3 connected to rotate with the turbine shaft and carrying a fixed contact Ill and a movable contact H5. Connection is made to the contacts I and III by means of slip ring contacts or brushes H0 and H1, one of which is connected to the other terminal of the winding IIO by a conductor I I0, and the other of which is connected to the other terminal of the heating element I01 by a conductor H9. The arrangement is such that the contacts Ill and H5 will close, under the influence of centrifugal force, when the speed of the turbine 20 reaches a predetermined safe maximum. The circuit will then be completed causing the secondary winding IIO to energize and heat the element I01, the heat from which immediately affects the thermally sensitive resistance 00 causing it to increase in effective resistance. Y

It may here be noted that before described secondary windings 00, I0. 00. 00, and H0 may be arrangedion a common transformer, which may be the transformer 00, but they are here shown transformers, having separate pimary windings represented at I20, I2I, I22, I23 and I respectively. In any event, the various primaries will be connected to acommon source of alternating current potential so that the alternating poten-' tials at the respective secondary terminals will at all times be in phase with each other. The signal potential impressed across the input terminals II and 02 of the the algebraic sum of a number of potentials developed in the respective networks 04, A, 50, and

Operation of Figure 1 For convenience in this description, the potential conditions existing during a half cycle at which the various transformer secondary windings have the polarities indicated by the legends in the drawing will only be considered. For a reference potential, the conductor 59 is indicated as grounded at I20, so that the ampliiier input terminal 52 is at zero potential to ground.

Atthe outset, the network A will also be assumed to be in balance so that it introduces no signal potential into the circuit between the networks 00 and 00, and conductors 55 and B are considered as of the same potential.

Considering then the network 58, the slider in the position shown is slightly positive with respect to the center tap on the secondary winding 00, while the slider 10 being at a midpoint along the resistance 01 is at a substantial negative potential with respect to said center tap. This network is thus seen to introduce a potential into the seriescircuit connecting the amplifler input terminals of such polarity that the slider 10 and conductor 01 are negative with respect to the grounded conductor 59.

Turning now to the network 50, it will be evident that with the sliders 11 and 86 in the position shown, this network introduces into the aforesaid series circuit a voltage equal to the potential of the slider 11 with respect to the left hand terminal of the secondary winding I3. This potential is obviously such that the slider 06 is positive with respect to slider 11 and the potential appearing at the slider 00 with respect to ground depends upon the relative magnitude of the respectively negative and positive potentials introduced by the networks 58 and 56. For convenience, it may be assumed that the magnitude of the unbalance voltage of network 10 is greater than that of net-.

work 50 so that the slider 06 is at a positive potential with respect to grounded conductor 09.

At this point, it is considered that the slider 06 and conductor 55 are at the same relative potential and the network A is'ignored as stated for the time being.

Considering finally the network 54, it will be evident that with the slider 04 at the midpoint. indicated upon the resistance 9|, it is at a negative potential with respect to conductor 50 so that this network introduces a signal voltage into the series circuit which opposes that between slider 00 and ground. For convenience, it may be assumed that the voltage introduced by network 54 exactly opposes the voltage between slider 00 and ground.

Under these assumed conditions then, the amplliier input terminal II is at the same ground potential as is the other input terminal 52 and all of the hereinfor convenience as separate I amplifier 45 will thus be no signal is impressed across the input of the amplifier. Under such conditions, the amplifier, course, supplies no current-to waste gate motor 34 and the waste gate remains at rest in the position called for by adjustment of the pressure selector 35 and other variable factors in the series circuit. However, should any one of the sliders in the various networks be moved from the positions here shown. it will be apparent that there will immediately occur a signal potential or difference in potential between the amplifier input terminals i and 32, due to the unbalance thus set up in the series circuit. The amplifier will immediately respond to this signal potential to supply the motor 34 with an ener izing current in the field winding 33 such as to cause the motor 34 to move the waste gate 23 in a direction depending upon the polarity of said signal potential, until displacement of the rebalancing potentiometer 94 rebalances the bridge circuit.

For example, consider the result of a rise in pressure in the engine induction system. The bellows 33 is expanded by such pressure rise and the slider 11 is moved to the left along the resistance 14 reducing the magnitude of the positive voltage introduced by network 55 in the series circuit. This positive voltage is thus made insufiicient to cancel out the negative voltages introduced by networks 54 and 53, whereupon the slider 94 becomes negative with respect to grounded conductor 59 causing the appearance at the amplifier input of a signal potential of a polarity such that terminal 5| is negative with respect to terminal 52. In terms of alternating voltage, this means that an alternating signal potential of a predetermined phase relation with respect to the supply voltage is supplied to the amplifier. Assuming that the amplifier 45 responds to a signal of this phase to provide motor field winding 35 with current oi! the proper phase, the motor will then move the waste gate 23 toward open position, reducing the pressure of the 2xhaust gases upon the turbine 25 and decreasing the speed thereof to reduce the compression ratio of the compressor I! to the proper point to compensate tor the aforesaid rise in induction system pressure. Movement of the waste gate 25 is accompanied by the movement of the slider 94 to the left along resistance 9i decreasing the volt age between slider 94 and conductor 55. In terms of the single half cycle, this means that the slider 94 is less negative than conductor 55 so that conductor 53 and input terminal 5i approach the same potential as the grounded conductor 59. This rebalancing movement of the slider 94 continues until the amplifier input terminal 5| assumes the same potential as input terminal 52 whereupon the motor 34 stops, leaving the waste gate 23 in a new position and with the series circuit again balanced.

In the same fashion a decrease in induction system pressure will reverse the movement of the slider 71 causing the appearance of a positive signal potential in the amplifier during the assumed half cycle, In terms of alternating current this means that there is applied to the ampliher an input voltage of opposite phase to that previously applied so as to cause the waste gate 23 to be moved toward closed position until the series circuit is again rebalanced by the coincident movement of the slider 94.

The same unbalancing, and then rebalancing, actions occur in the series circuit upon the movement of any of the other sliders 64, 13 and 33 and when the system functions properly, it will be apparent that any desired value of induction system pressure may be selected by the controller 55 and automatically maintained within close limits. The function of the controller 33 is, of course, to limit the rate of acceleration of the turbo-supercharger and when the acceleration reaches the predetermined safe rate, this slider is moved to the right along the resistance 33 to introduce a signal potential such as to cause opening movement of the waste gate to retard the speed of the turbine.

Considering now the operation of the network A. it is first assumed that the resistances 91 and 93 normally are of such values that, with the slider I35 properly positioned, this network will be balanced, that is, the conductor B will be at the same potential with respect to the terminal of the secondary winding 93 as is the conductor 55. As heretofore stated, however, the resistances 91 and 93 each have a high temperature coefilcient of resistance so that when heat is applied they will rise in efiective resistance in proportion to their operating temperatures.

The centrifugally operated switch I I2 is so ad- J'usted that when the turbine 25 and its shaft 29 reach a predetermined speed, the movable switch contact H5 will be thrown outward into contact with the fixed contact H4, thus completing a circuit from the transformer secondary winding H0 to the heating element I01. This circuit may be traced from the upper terminal of the winding H3 through the conductor H8, the brushes H3 and H1 across which the circuit is closed by the contacts H4 and H5, through the conductor 9 to the heater element I01, and through the conductor ill back to the lower terminal of the secondary winding. When this circuit is thus closed, the heater element I01 immediately rises in temperature and, bein enclosed along with the resistance 98 within the insulating enclosure I33, this rise in its temperature is immediately eifective upon said resistance. As the resistance 93 is thus heated, it rapidly increases in resistance, having the effect oi. shifting the conductor B toward the left hand terminal of the secondary winding 96 or causing it to become more nearly the same potential as that terminal. Considering the operation, as heretofore stated, during a half cycle at which the polarity is as indicated by the legends in Figure 1, it will be apparent that this increase in resistance causes the conductor 55 to become relatively negative with respect to conductor B. There thus is introduced an unbalanced condition such that a negative signal potential is introduced into the series circuit causing the amplitying input terminal 5! to become negative with respect to terminal 52 and, as aforesaid, causing the amplifier to so energize the field winding 35 as to cause the motor 34 to move the waste gate 23 toward open position. This action has the immediate efl'ect of reducing the speed of the turbine so that this thermally responsive network A thus acts as a true overspeed control.

The resistance 9'! acts to provide ambient temperature compensation, Any change in the amblent temperature affects resistances 91 and 93 equally so as not to disturb the balance of the bridge.

When the turbine speed is thus reduced and maintained at less than a predetermined maximum value, the switch III of course will open so that the circuit to the heater element I3! is broken. However, the enclosure of this heater element along with the resistance 98 within the heat insulating body I88 retards the cooling of these parts so that the network A approaches a balanced condition slowly with respect to the speed at which it becomes unbalanced when an overspeed condition occurs in the turbine. Thus the waste gate opening signal is removed quite slowly and to such point that the call of the balance of the control system for a high turbine speed will not result in the waste gate being moved back to closed position. This would tend to cause rapid operation of the turbine with the result of the switch II2 being immediately reciosed, causing a rapid hunting in the control system.

While I have shown a'switch operated by the centrifugal mechanism II2, it is to be understood that the centrifugal mechanism might operate a rheostat in series with heater I81 to provide for a variable degree of energization of the heater. This would result in a modulatin effect of the overspeed controller instead of an on-and-ofl operation as provided by the switch.

The adjustable slider I85 permits the network A to be balanced under any initial conditions, as will be apparent.

Fromm 2 In Figure 2, I have illustrated a preferred mechanical and electrical layout of the thermally responsive elements of the network A of Figure 1.

Here the resistances 91 and 98 are wound on identical tubular forms I28 and I21 which are mounted on conventional vacuum tube bases I28-I29 and surrounded by vacuum tube envelopes I38-I3I. These resistances 91 and 98 are wound around resistance wires I32 and I33 respectively which are insulated electrically from the resistances 91 and 98 and which are adapted to heat the resistances 91 and 98. The terminals of the heater wire I32 are brought down as indicated at I34 and I35 to two pins I38 and I31 on the base I28, while the terminals of the resistance wire 91 are brought down as indicated at I38-I39 to another pair of pins I48--I H on the base. The terminals of the heater wire I33 are brought down as indicated at I42-I43 to pins I44I45 on the base I29 which pins correspond exactly to the pins I35I31 on the other base I28. In similar fashion, the ends of the resistance wire 98 are brought down as indicated at I48-I41 to pins I48-I49 corresponding exactly in position to the pins I48--I4I on the other base I28.

The two identical units thus made are enclosed within an outer casing I58 having a sub-base II in which two conventional vacuum tube sockets I52 and I53 are arranged in spaced relation; The two vacuum tube-like units may thus be plugged into the sockets I52 and I53 interchangeably, but to the socket I52 only two conductors I54 and I55 will be connected, making connections to the pins I48 and I so that whichever unit is plugged into the socket will have its heater left floating or unconnected. The conductors I54 and I55 will, of course, make connection to the conductors 99 and MI appearing in Figure 1. On the other hand, the socket I53 has conductors I58I51 making connection to the pins I44-I45, and another pair of conductors I58-I59 making connection to the pins I48I49. Thus, the unit plugged into the socket I53 will have connections made both to its resistance and to its heater. In the arrangement herein shown, the conductors I58-I59 make connections at I88 and Ill in Figure 1, while the conductors I58 and I 51 make connections to the heater circuit conductors III and H8.

It will be apparent from two units thus formed. and with the socket con;

nection described. are completely interchangeable, and should the heater element in one burn out then the other may be substituted to keep the network in operative condition. The resistances 91 and 98 are, of course, identical in characteristics since they are used in opposite legs of the bridge.

The envelope I3I of the unit in which the heater element is operative serves the same purpose as the heat insulating enclosure indicated at I88 in Figure l, causing a temperature rise to rapidly effect the resistance 98, while the enclosure of both units in the casing or housing I58 subjects the unit acting as the resistance 81 to the ambient temperatures about the other heated unit for the purposes previously set forth. The casing I58 at the same time acts to dissipate and radiate the heat developed to such extent as to prevent the unit carrying resistance 81 from becoming heat saturated to a point at which the two resistances might assume the same value to .,cause undesired balancing of the bridge.

The interiors of the vacuum tube envelopes I38-I3I are filled and sealed off with hydrogen, nitrogen or other inert gas which has the property of being a poor heat conductor and, when enclosed as it is here, of constant density. The use of a gas for this purpose will retard the conduction of heat from the heating element or wire I93 so that the bridge will rebalance slowly upon the disconnection of the heater from the energizing transformer. cooling and rebalancing action is desirable to prevent hunting in the control system. The kind of gas used will affect the heating and cooling cycles of the bridge and may be selected in accordance with requirements. The selected thermal characteristics may be maintained regardless of altitudes if a gas of suitable properties is chosen.

Fro-oar: 3

such parts correspond they are given correspondv ing reference numerals in Figure 3.

In the network 54, moreover, the transformer secondary winding 98 has a tap I88 and the slidewire resistance 9| is connected at one end by a conductor I8I to this tap and at its other end by a conductor I82 to the right hand terminal of the winding, being thus placed across only a part of the winding in this case.

Another slidewire resistance I63 is provided and it is connected between the tap I88 and the left hand terminal of the secondary winding 98 by conductors I84 and I85. Cooperating with this resistance I83 is a slider I88, to which conductor B is connected, and together the resistance and slider form an overspeed controller designated generally at I61. This controller is thermally actuated as will now be described.

The slider I88 is carried and positioned by one end of a bimetal strip I88 anchored at its other end to a suitable support as indicated at I89. Around this strip I88 a heating element I18 is the foregoing that the- As stated heretofore, this slow 11 coiled and, together with the strip, is enclosed in a flexible insulating sheath I1I. ;;Said heating element is connected through the conductors III and III to the transformer secondary IIII under the control 01' the switch II2, thus replacing the heater element I01 01' Figure 1. The entire controller I61 is enclosed with a heat dissipating housing or enclosure I12 and the resistance I63 is movably supported by the free ends of a pair 01' bimetal strips I13-I14, the opposite ends of which are anchored at I'|5I16. The strips I1l-I14 are exposed to ambient temperatures within the-enclosure I12 and about the heater I16.

Operation of Figure 3 As here shown, and still assuming the polarity to be as indicated by the legends in the drawing, the slider 94 is negative with respect to the slider I66 by a magnitude determined by the value of the resistance 9i between the slider 94 and the conductor I62 and the value of resistance I63 between slider I66 and conductor I64. The system may obviously be so adjusted that, while the network 54 now introduces a negative signal voltage into the series circuit, it is balanced out by the sum of the voltage introduced by .the balance of the network, so that no signal is preseat at the amplifier input.

Now if the speed of the turbine starts to exceed a safe maximum value, causing the switch contacts H4 and 5 to close, then the heater I will be energized and the uneven expansion of the bimetal strip I66 due to the resulting temperature rise will cause the strip to flex or bend, moving the slider I66 along the resistance I63. Properly arranged, this movement of the strip I68 will so adjust the slider I66 as to unbalance the network 54 and cause a signal potential to be developed across the amplifier input terminals 5| and 52 of a polarity such as to cause the energization of the motor field winding 36 and rotation of the motor 34 in proper direction to cause opening movement of the waste gate 28. For example, the movement of the slider I66 may be to'the left in Figure 3 so that slider 94 becomes relatively more negative and a signal potential is developed such that the amplifier input terminal Si is negative with respect to grounded input terminal 52. The opening movement of the waste gate 28 will of course counteract the overspeeding tendency of the turbine.

The heat insulation I1I about the bimetal strip I60 causes the same to heat very rapidly, upon energization of the heater I10; conversely, it retards cooling so that the waste gate will not be immediately reclosed by rapid cooling when the switch II2 opens in response to the decrease in speed called for by the initial heating. Thus the efl'ect is to cause a thermal lag such as to remove the waste gate opening signal more slowly than It is introduced. This is of advantage in counteracting any hunting tendencies in the control.

The bimetallic strips I13 and I14 act to provide ambient temperature compensation. Upon any change in ambient temperature, the bimetallic elements I66, I13 and' I14 are afiected equally. While the eflect of the temperature on bimetallic element I66 is to tend to cause movement or slider I66 with respect to resistance I63, the bimetallic elements I13 and I14 at the same 12 time tend to move resistance III in the same direction. The result is that there is no relative movement between the slider I66 and resistance I63. When heater I10 is energized, however, only bimetallic element I66 is heated.

It is understood that suitable modifications may be made in the structure as disclosed, provided such modifications come within the spirit and scope of the appended claims, Having now therefore fullyillustrated and described my invention, what I claim to be new and desire to protect by Letters Patent is:

I claim as my invention:

1. An electrical control system for a variable velocity device, comprising in combination,- a voltage responsive control means for said device .Ior varying the velocity of said device, means including an electrical impedance means connected to said control means for varying the voltage applied to said control means in accordance with the impedance of said impedance means, thermally operated means responsive to an increase in velocity of said device above a predetermined maximum to vary the impedance of said impedance means at a rapid rate and apply to said control means a voltage operative to cause said control means to reduce the velocity, said thermally operated means being thermally lagged to retard the rate at which the impedance value of said impedance means is returned to its normal value as said velocity diminishes.

2. Electrical apparatus for controlling a variable condition, comprising in combination, control means for varying said condition to maintain a desired value of said condition, a normally balanced compound electrical network connected to said control means and operative when unbalanced for any cause to actuate said controlmeans for varying the condition, said network including a normally balanced thermally responsive portion, and means responsive to a predetermined variation in said condition for thermally actuating said network portion to unbalance the network and correct the condition, and said network portion being thermally lagged in only one direction to retard the rate at which the network is restored to balance when said predetermined variation in the condition is corrected.

3. Electrical apparatus for controlling a variable condition, comprising in combination, control means for varying said'condition, a normally balanced compound electrical network connected to said control means and operative when unbalanced for any cause to actuate said control means for varying the condition to maintain said condition at a desired value, said network including a normally balanced bridge embodying at least one resistance element having a high temperature coefficient of resistance, heating means operative in response to a predetermined change in the condition for heating said resistance element to cause the same to vary in effective resistance and thereby unbalance the network and initiate the correction of the condition, and means for retarding only the rate of cooling of said element when the condition is corrected so that the unbalanced condition in the network will disappear more slowly than it was introduced in response to said change in the condition.

4. Electrical apparatus for limiting the velocity of a variable velocity device, comprising in combination, an electronic amplifier responsive to a signal potential for varying the velocity 01' said device, a normally balanced electrical network operative when unbalanced to produce a signal potential for actuating said amplifier. and thermally operated means in said network responsive to an increase in the velocity of said device above a predetermined maximum for rapidly introducing an unbalancing efiect into the network and producing a signal potential calling for a reduction in said velocity and then gradually removing the unbalancing eflect as the velocity diminishes below said maximum.

5. Electrical apparatus for limiting the velocity of a variable velocity device, comprising in combination, an electronic amplifier responsive to a signal potential forvarying the velocity of said device, a normally balanced electrical network operative when unbalanced to produce a signal potential for actuating said amplifier, and thermally operated means embodied in the network responsive to an increase in the velocity of said device above a predetermined maximum for introducing an unbalancing effect into the network and producing a signal potential calling for a reduction of said velocity, said meansbeing thermally lagged to cause said unbalancing efiect to disappear slowly as the velocity of the device is reduced.

6. Electrical apparatus for limiting the velocity of a variable velocity device, comprising in combination, an electronic amplifier responsive to a signal potential for varying the velocity of said device, a normally balanced electrical network operative when unbalanced to produce a signal potential for actuating said amplifier, thermally operated means embodied in the network responsive to an increase in the velocity of said device above a predetermined maximum for temporarily unbalancing the network and calling for a reduction in velocity of said device, and another thermally responsive means subject to ambient temperatures about the first mentioned means and operative to compensate for changes in ambient temperature.

7. Electrical apparatus for controlling a motordriven supercharger, comprising in combination, means for adjusting the velocity of the motor driving said supercharger, means responsive to a signal potential for controlling said adjusting means, a normally balanced electrical impedance network operative when unbalanced to produce a signal potential effective on the said last named means, means responsive to a pressure condition affected by said supercharger for varying the balance of the network, and means including said network responsive to the velocity of said motor and operable to unbalance the network when a predetermined safe velocity has been exceeded to produce a signal potential calling for a reduction in said velocity and then to rebalance the network as the velocity diminisheasaid last named means having a thermally responsivev element, a heating element associated therewith,

and means responsive to the velocity of said motor for controlling the energization of said heating element.

8. Electrical apparatus for controlling a motor driven supercharger, comprising in combination, means vfor adjusting the velocity of the motor driving said supercharger, an electronic amplifier responsive to a signal potential for controlling said adjusting means, a normally balanced compoundv electrical network operative when unbalanced to produce. a. signal potential eflectiveon the amplifier, means responsive to a pressure condition. afiectedby said supercharger for varying the balance of .the network, and means including a. bridge making up apart ot said network ro sponsive to an increase in the velocity of the motor to a predetermined safe maximum to quickly unbalance the network and produce a signal potential calling for a reduction in said velocity and then to gradually rebalance the network as the velocity diminishes, said bridge including a plurality of legs and two thermally responsive resistance elements in different lels thereof, a heating element directly associated with one resistance element, and means for said heater under control of the motor velocity to unbalance the network, the other of said resistance elements being exposed to ambient temperatures about the first and operative to compensate for changes in ambient temperature.

9. Electrical apparatus for controlling a motor driven supercharger, comprising in combination, means for adjusting the velocity or the motor driving said supercharger, an electronic amplifier responsive to a signal potential for controlling said adjusting means, a normally balanced electrical network operative when unbalanced to produce a signal potential effective on the amplifier, means responsive to a pressure condition affected by said supercharger for varying the balance of the network in response to variations in said pressure condition, and further meansin said network responsive to an increase in the velocity of the motor to a predetermined safe maximum to quickly introduce an unbalancing efiect into the network and produce a signal potential calling for a reduction in said velocity and then to gradually rebalance the network as the velocity diminishes, said further means including at least one thermally responsive resistance, a heater associated therewith, and means responsive to an increase in velocity of the motor to energize said heater to unbalance the bridge, said resistance and heater being heat insulated to retard cooling as the velocity diminishes.

10. Electrical apparatus for controlling a motor, comprising in combination, means for adjusting the velocity of the motor, an electronic amplifier responsive to a signal potential for controlling said adjusting means, a normally balanced electrical impedance network operative when unbalanced to produce a signal potential effective on the amplifier, and means in said network responsive to an increase in the velocity of the motor beyond a predetermined safe maximum to quickly unbalance the network and produce a signal potential calling for a reduction in said velocity and then to gradually rebalance the network as the velocity diminishes, said last named means in-" cluding at least one thermally responsive resistance, a heater connected in said network, and means responsive to an means for adjusting the velocity of said device,

a normally balanced electrical network connected to said control means and operative when unbalanced to actuate said controlmeans to vary the velocity of said device, said network including two thermally responsive resistances exposed to the same ambient temperature and connected"!!! opposing portionsoi said network, a heater assoincrease in velocity of said motor to energize said heater, said resistance eiat'ed with one of said resistances, and means responsive to the velocity oi said variable velocity device for energizing said heater to unbalance said network, said two thermally responsive resistances being enclosed within enclosure means having plug-in base means, and said apparatus including socket means in which said base means is detachably secured.-

12. Electrical apparatus for controlling a variable velocity device, comprising in combination, means for adjusting the velocity of said device, a normally balanced electrical network connected to said control means and operative when unbalanced to actuate said control means to vary the velocity oi said device, said network including two thermally responsive resistors, said two thermally responsive resistances being each enclosed within a separate enclosure having a plugin base, said apparatus including a pair of sockets in which said bases are interchangeably mounted, said bases and sockets having connections associated therewith for connecting said resistors into said network in such a manner that equal changes in the resistance oi said resistors do not affect the balance of said network, a heating element associated with each or said resistors, and means responsive to the velocity of said device for establishing a connection from a source of energy upon the velocity exceeding a predetermined value, the connections associated with one only oi. said sockets providing for "connection oi said last named means to the heater 0! the resistance-unit whose plug-in base is plugged into that socket.

13. Electrical apparatus for controlling a variable condition, comprising in combination, control means for varying said condition, anormally baianced compound electrical network connected to said control means and operative when unbalanced for any cause to actuate said'control means for varying the condition, said network including a potentiometer type resistance and slider, thermally operated means responsive to a predetermined variation in said condition for adjusting said slideralong the resistance in one direction and for thereby unbaiancing the network to correct the condition, and thermally operative means operative to displace the resistancerelative to the slider under the influence of ambient tempera.- tures developed by the first mentioned thermally operated means and in av direction such as to initiate a rebalancing eflect in the network.

GEORGE L. BOB-ELL.

REFERENCES CITED.

The following references are of record in the me of this patent:

UNITED STATES PATENTS Number Name Date 1,210,049 Copley Dec. 26, 1916 1,767,052 Grouse et a1 June 24, 1930 1,895,498 Stoller Jan. 31, 1933 2,205,306 Olshevsky June 18, 1940 2,264,487 Smulski Dec. 2, 1941 2,376,142 Hoifmann et al May 15, 1945 

